There are known techniques for detecting foreign objects in a liquid.
For instance, in the field of droplet discharging devices, there are proposals of techniques for detecting whether a bubble or other foreign object exists in the liquid to be discharged.
In such techniques, the presence or absence of a bubble is determined from e.g. the attenuation of the vibration level due to the existence of a bubble and the residual vibration waveform.
However, in such techniques, it is difficult to detect a bubble or other foreign object in a small region such as a thin duct. Thus, there is demand for the development of techniques capable of accurately detecting a bubble or other foreign object in a small region such as a thin duct.